Exotics Searches at ATLAS

An overview of recent searches for exotic signatures using the ATLAS detector at the LHC is given. The results presented use data collected at center-of-mass energies of $\sqrt{s}$ = 7 TeV and $\sqrt{s}$ = 8 TeV, for datasets corresponding to a variety of integrated luminosities. Resonance searches using leptons, photons, missing transverse energy, and jets are performed, as well as searches requiring custom jet and track reconstruction. No deviations from Standard Model expectations are observed.Comment: 7 pages, Workshop on high energy physics in the near future, Rio de Janeiro, Brazil, 17 - 24 Mar 201

Improved γ\gamma/hadron separation for the detection of faint gamma-ray sources using boosted decision trees

Imaging atmospheric Cherenkov telescopes record an enormous number of cosmic-ray background events. Suppressing these background events while retaining $\gamma$-rays is key to achieving good sensitivity to faint $\gamma$-ray sources. The differentiation between signal and background events can be accomplished using machine learning algorithms, which are already used in various fields of physics. Multivariate analyses combine several variables into a single variable that indicates the degree to which an event is $\gamma$-ray-like or cosmic-ray-like. In this paper we will focus on the use of boosted decision trees for $\gamma$/hadron separation. We apply the method to data from the Very Energetic Radiation Imaging Telescope Array System (VERITAS), and demonstrate an improved sensitivity compared to the VERITAS standard analysis.Comment: accepted for publication in Astroparticle Physic

Optical Microlensing by Primordial Black Holes with IACTs

Primordial black holes (PBHs), hypothesized to be the result of density fluctuations during the early universe, are candidates for dark matter. When microlensing background stars, they cause a transient apparent enhancement of the flux. Measuring these signals with optical telescopes is a powerful method to constrain the PBH abundance in the range of $10^{-10}\,M_{\odot}$ to $10^{1}\,M_{\odot}$. Especially for galactic stars, the finiteness of the sources needs to be taken into account. For low PBH masses (in this work $\lesssim 10^{-8}\,M_{\odot}$) the average duration of the detectable event decreases with the mass $\langle t_e\rangle \propto M_{\mathrm{PBH}}$. For $M_{\mathrm{PBH}}\approx 10^{-11}\,M_{\odot}$ we find $\langle t_e\rangle \lesssim\,1 \mathrm{s}$. For this reason, fast sampling detectors may be required as they could enable the detection of low mass PBHs. Current limits are set with sampling speeds of 2 minutes to 24 hours in the optical regime. Ground-based Imaging Atmospheric Cherenkov telescopes (IACTs) are optimized to detect the $\sim$ns long optical Cherenkov signals induced by atmospheric air showers. As shown recently, the very-large mirror area of these instruments provides very high signal to noise ratio for fast optical transients ($\ll 1\,$s) such as asteroid occultations. We investigate whether optical observations by IACTs can contribute to extending microlensing limits to the unconstrained mass range $M_{\mathrm{PBH}}<10^{-10}M_\odot$. We discuss the limiting factors to perform these searches for each telescope type. We calculate the rate of expected detectable microlensing events in the relevant mass range for the current and next-generation IACTs considering realistic source parameters.Comment: 8 pages, 3 figures, Proceedings of the 37th International Cosmic Ray Conference (ICRC 2021), Berlin, German

A VERITAS/Breakthrough Listen Search for Optical Technosignatures

The Breakthrough Listen Initiative is conducting a program using multiple telescopes around the world to search for "technosignatures": artificial transmitters of extraterrestrial origin from beyond our solar system. The VERITAS Collaboration joined this program in 2018, and provides the capability to search for one particular technosignature: optical pulses of a few nanoseconds duration detectable over interstellar distances. We report here on the analysis and results of dedicated VERITAS observations of Breakthrough Listen targets conducted in 2019 and 2020 and of archival VERITAS data collected since 2012. Thirty hours of dedicated observations of 136 targets and 249 archival observations of 140 targets were analyzed and did not reveal any signals consistent with a technosignature. The results are used to place limits on the fraction of stars hosting transmitting civilizations. We also discuss the minimum-pulse sensitivity of our observations and present VERITAS observations of CALIOP: a space-based pulsed laser onboard the CALIPSO satellite. The detection of these pulses with VERITAS, using the analysis techniques developed for our technosignature search, allows a test of our analysis efficiency and serves as an important proof-of-principle.Comment: 15 pages, 7 figure

VERITAS discovery of very high energy gamma-ray emission from S3 1227+25 and multiwavelength observations

We report the detection of very high energy gamma-ray emission from the blazar S3 1227+25 (VER J1230+253) with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). VERITAS observations of the source were triggered by the detection of a hard-spectrum GeV flare on May 15, 2015 with the Fermi-Large Area Telescope (LAT). A combined five-hour VERITAS exposure on May 16th and May 18th resulted in a strong 13$\sigma$ detection with a differential photon spectral index, $\Gamma$ = 3.8 $\pm$ 0.4, and a flux level at 9% of the Crab Nebula above 120 GeV. This also triggered target of opportunity observations with Swift, optical photometry, polarimetry and radio measurements, also presented in this work, in addition to the VERITAS and Fermi-LAT data. A temporal analysis of the gamma-ray flux during this period finds evidence of a shortest variability timescale of $\tau_{obs}$ = 6.2 $\pm$ 0.9 hours, indicating emission from compact regions within the jet, and the combined gamma-ray spectrum shows no strong evidence of a spectral cut-off. An investigation into correlations between the multiwavelength observations found evidence of optical and gamma-ray correlations, suggesting a single-zone model of emission. Finally, the multiwavelength spectral energy distribution is well described by a simple one-zone leptonic synchrotron self-Compton radiation model.Comment: 18 pages, 6 figures. Accepted for publication in the Astrophysical Journal (ApJ

VizieR Online Data Catalog: Mrk421 in March 2010 (Aleksic+, 2015)

I. The multi-wavelength light curves (LCs) of Mrk 421 between MJD 55264 and 55278, from VHE to radio (the data in Fig. 1) are given in 32 files (INSTRUMENT_BAND.dat) II. The day-by-day broadband spectral energy distributions (SEDs) between MJD 55264 and 55278 (the data in Figs. 7,8a-9f,12a-13f) are given in 13 files (55265-55277.dat) (19 data files)

Measurement of inclusive jet charged-particle fragmentation functions in Pb plus Pb collisions at root S-NN=2.76 TeV with the ATLAS detector

© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/license/by/3.0/). Funded by SCOAP3